Jitter compensating circuit for angle encoding apparatus



4-, R. C. WEBB A JITTER COMPENSATINQ CIRCUIT FOR ANGLE ENCODINGAPPARATUS Original Filed March 4, 1 959 2 Sheets-Sheet 1 TransducerPhase Phase D Detecto\r letector e75 73 69 63 c 2 J 96 65 Fr egu encyJitter \IOOO Dmdmg 57 Jitter Com ensator Co nsator Controlled Controlledk Oscillator Oscillator 9 98 (Reference) (Variable) M \x? 79\) 7 8| 7Phase Comparator and Up-Down Separator 83 i 85 Zero Crossing ,89

Up Down Coincidence Detector Continuous 86 Reversible p /l Counter v J J'T J J J Degree Vernier INVENTORI RICHARD C. WEBB April 4, 1967 R. c.WEBB 3,312,903

JITTER COMPENSATING CIRCUIT FOR ANGLE ENCODING APPARATUS Original FiledMarch 4;, 1959 2 Sheets-Sheet 2 .137 FIG. 2

|NVENTOR= RICHARD C. WEBB United States Patent 3,312,903 JITTERCOMPENSATING CIRCUIT FOR ANGLE ENtIODlNG APPARATUS Richard C. Webb,Broomfield, Colo., assignor, by mesne assignments, to InternationalTelephone and Telegraph Corporation, New York, N.Y., a corporation ofMaryland Griginai application Mar. 4, 1959, Ser. No. 797,264, now PatentNo. 3,152,324, dated Oct. 6, 1964. Divided and this application Jan. 8,1963, Ser. No. 250,185 1 Claim. (Cl. 328-127) This invention is adivision of application, Ser, No. 797,264, filed Mar. 4, 1959, now US.Patent No. 3,152,324 assigned to the assignee of this invention. Theinvention relates to jitter compensating circuit use with apparatus andcircuitry used for the obtainment of a continuous measurement anddigital encoding of the relative phase difference between twosubstantially sinusoidal electrical signals of equal frequency butvariable phase angle.

In its preferred form, the invention finds a principal application inthe digital encoding of the relative phase difference between twoelectrical signal waves generated under the control of a precisionelectrostatic shaft-position-transducer which may be, illustratively, ofthe form described by this applicant in United States patent applicationSer. No. 578,853, filed Apr. 17, 1956, now US. Patent No. 2,930,033. Thetransducer in its preferred form is a generator of two sinusoidalelectrical signals of known and selected frequency, normally identicaland one of which is of reference phase while the other adjusts to anadvanced or retarded phase position in proportion to angular movementsimparted to the mechanical shaft. Some phase measuring devices of theabove type are designed to produce an electrical phase shift ofprecisely 360 electrical degrees for each mechanical degree ofmechanical movement. By this invention a continuous electronic phaseangle encoder is used in conjunction with an angle transducer toprovidean apparatus by which angular positions of a shaft are interpreted interms of digital signals with an accuracy considerably greater than thatachieved by methods heretofore known or suggested.

The techniques embodied in this invention avoid the need for sampling ofthe analog information to be encoded into digital form, as is common inmany analogto-digital encoding schemes, so that the digital informationis continuously available from the electronic apparatus and there is nodelay entailed in the acquisition of information, as has been inherentin encoders of earlier designs.

According to the principles of this invention a large number of.applications for shaft encoding apparatus of high precision manifestthemselves. Such applications, by way of example, may be in thecontinuous measurement of azimuth and elevation shafts in opticaltracking instruments and radars employed in guided missile rangeinstrumentation; the measurement of angular displacements of stableplatforms, the registration of angular settings of rotary tables used inmachine tool applications, as well as many other applications that willsuggest themselves to those skilled in the art to which the invention isdirected.

The broad principle involved in the electrical phase angle measuringdisclosed by the invention herein to be set fort-h is that of firstrelating each of the two signals Whose relative phase is to be measuredto selected harmonies of sufliciently high order to permit the harmonicfrequencies to be used as a time measuring scale of fractional portionsof the basic or fundamental signals.

In accordance with the invention and that preferred form of it herein tobe particularly discussed, the onethousandths harmonic of a given waveis used as a scale, which permits making measurements to a least countor granularity of of a wavelength or 0.360 electrical degree. As theinvention will herein be described, each of the two basic signals isrelated to a selected harmonic of the same order (the one-thousandthharmonic, for example) and these harmonic signals are applied to asumming circuit wherein they are permitted to interfere constructivelyand destructively (additively and subtractively) with one another inaccordance with movements or shifts in the relative phase position ofthe basic signals. In a case where the related signals are chosen as theone-thousandth harmonic, one thousand interference maxima .and minimawill occur as the basic electric waveforms are shifted in phase by onewavelength. If this amount of electrical shift is produced by a shaftpositiontransducer due to rotation through one mechanical degree, then asummation of interference maxima is a measure to one-thousandth part ofa mechanical degree or increments of 3.6 sec. of arc.

- In the event the mechanical shaft is rotated first in one directionand then in the other, that is, for instance, clockwise andcounter-clockwise, it becomes necessary to register the number of upcounts (as for an increasing phase change) .as well as the number downcounts (as, for instance, for a decreasing phase change) to maintain thelatest information concerning the shaft setting. One part of thisinvention relates to an up-down separator which functions in conjunctionwith the interference detector to transmit a series of up counts over asignal channel to a digital counter, as well as a series of down countsover another signal channel to the same counter, the counter being oneof a type adapted to be advanced by up counts and retarded by downcounts, thereby retaining at all times a net count at any given instant.

Mechanical transducers of the foregoing type, even when made to theultimate limit of mechanical perfection, inherently introduce amplitudeand phase perturbations in the two basic signals derived therefrom.These mechanical imperfections result in a so-called jitter in theproduced results unless compensated but by this invention they areminimized and substantially compensated for by purely electrical meansaccording to the circuitry herein to be described. The mechanicaltransducers simultaneously produce both the amplitude and phasecomponents of the mechanical perturbation. Fortunately, only the phaseerrors register in the electronic encoding apparatus which issubstantially insensitive to amplitude disturbances. Accordingly, itbecomes possible to amplitude dem-odulate the perturbation wave toproduce a signal voltage that is substantially a replica of thedisturbance. This invention provides for recovering such a voltage whichis then introduced into an appropriate electrical phase-locking circuitto be used to tie the high frequency harmonic to its base signal in sucha manner as to introduce a counter-perturbation into the phaselockingcircuit in such a way as to oppose the natural perturbation comingdirectly from the phase modulation of the base signal. The combinedeffect is a neutralization of the disturbance.

This application is directed particularly to the electrical circuitry bywhich the jitter, as above defined, is compensated. The invention andcircuitry here disclosed provide a suitable control to aid indisplaying, with greatly improved accuracy, decimal displays ofmechanical conditions in an angle measuring assembly. In this respect,the invention is illustrated in a preferred form by the accompanyingdrawings which schematically show the J complete operation of thecircuitry and such associated components as are considered necessary toa full understanding of what is involved.

With the foregoing in mind it becomes apparent that one of the manyobjects of the invention is that of eliminating jitter in systems whichinhibit and record the angular position of a monitored device.

Other and further objects and advantages of the invention are set out bythe parent application above identified and still other objectives willbecome apparent and at once suggest themeselves to those skilled in theart to which the invention is directed in the following description asconsidered particularly in connection with the accompanying drawingswherein:

FIG. 1 is a diagrammatic view presented to show in block diagram formthe component parts of a phase comparison system using the inventivejitter compensator; and

FIG. 2 is a circuit diagram of a jitter compensating circuit forcompensating for changes in the generated wave forms.

For the purpose of illustrating the present invention in one of itsaspects FIG. 1 shows, for instance, that an electro-mechanicaltransducer 51 provides the wave generating mechanism and that regardlessof the built-in mechanical precision certain changes produce slowlyfluctuating amplitude of the generated voltage waves. This transducerhas a driven shaft element 11 which is turned continuously and uniformlyat as constant a speed as possible by means of a suitable driving motor(not shown but connected in any desired way to drive the shaft 11). Themotor is supplied with input motive power from any desired source (notshown) connected to it in well-known fashion.

In applying the invention to apparatus for encoding interference effectswhich constitute a measure of angular departures of one element relativeto another, the driven elements, a rotor which turns relative to a pairof stator elements (not shown herein but described in detail in theparent application). One of the stator elements remains in fixedposition at all times and the other is adjustable. The adjustable statorelement is subject to being changed in its angular position relative tothe rotor with the changes resulting from induced mechanical turnings ofa controllable component, such as the conventionally represented controlshaft 22 (FIG. 1). In each instance the rotor elements revolvinginternally of the stator elements provide output electrical waves ofsubstantially sinusoidal form whose frequency is substantiallyidentical, although the phase of the generated waves produced by therotor-stator combination which is adjustable is subject to changerelative to the reference frequency from the other wave-generatingcomponent. In this sense the phase may be advanced or retarded relativeto the reference phase.

In practicing the invention the element whose angular state is to bedetermined is rotated (as indicated by the arrows in FIG. 1) to turn theshaft 22 and the movable stator. This introduces a phase change in thewaveform available on conductor 55 as compared to that available onconductor 53. This change is then precisely determined.

It will be noted that for the purpose of indicating the fact that thevariable phase generated wave may either be retarded or advanced withrespect to the reference that the rotary transducer is shown as capableof revolving in either a clockwise or counter-clockwise direction, asdesignated by the arrows adjacent to the shaft 22 in FIG. 1. Outputsignal wave voltages provided on the conductors 53 and 55, the waveformson the conductor 53 for reference purposes being considered as developedfrom the combination of the permanently fixed stator and those waveformsdeveloped on the conductor 55 being considered to have been developed bythe relative change between the rotor and the movable stator element,the relationships, of which are adjustable with respect to each other,as already explained. The frequency of the output voltages on conductors53 and 55 may be chosen at any value desired, although it will beunderstood that the chosen generated frequency is determined by thenumber of separate pole pieces on the rotor and stator elements and thespeed of rotation of the elements.

The oscillators 57 and 59 are not shown by circuit components sincethese are of any form well known in the art. One form of the oscillatorwhich has proved quite satisfactory is the well-known Colpitts-typewhich provides extremely stable operation. The generated frequency is,however subject to control, as desired, in well-known fashion. Thereference and variable phase output voltages are developed and availablealong the conductors 53 and 55 from which they are supplied to suitablephase detectors 61 and 63, the character of which is well known and neednot be described herein in detail. Suffice it to say that the phasecomparison of voltages is made between a sub-harmonic of the referenceand variable master oscillator frequency as supplied to the phasedetectors from frequency dividers 65 and 67 through conductors 69 and71.

The frequency dividers 65 and 67 are of any wellknown type suitable forproviding a frequency division. In the illustrated case a frequencydivision of one thousand has proved satisfactory so that in the phasedetectors 61 and 63 a phase comparison is made between the mastercontrol oscillator at its one-thousandth sub-harmonic and the developedgenerated frequency. The technique followed in this phase detector isessentially similar to that adopted in the well-known form of televisioncircuitry wherein the so-called fly wheel type of synchronization isestablished. Any phase differences between the frequency of the waveformapplied via the conductors 69 and 53 to the phase detector 61 or via theconductors 71 and 55 to phase detector 63 will then manifast themselvesat a suitable control voltage available respectively on the conductors73 and 75 which then may be supplied as a DC. control voltage, forinstance, which is fed back to control and adjust the frequency of theoscillators 57 and 59, respectively. This control may be provided inwell-known manner, such as by applying the phase-detected voltage as again control signal for the usual form of reactance tube whose outputcontrols the magnitude of either an inductive or capacitive component inthe oscillatory circuit of the oscillators 57 or 59, depending uponwhich form of signal channel is to be selected. In this way an automaticfrequency control loop is established between the oscillators 57 and 59and the voltages developed on the conductors 53 and 55 through theaction of the phase detectors 61 and 63, as the case may be.

Output voltages from the oscillators 57 and 59 comprising the master andvariable control voltages occurring at a harmonic frequency of thatdeveloped by the transducer are also supplied by way of conductors 77and 79 to a phase comparator and so-called updown separator 81, whichhas been shown in block form in FIG. 1 but which circuitry is furtheroutlined and described in the parent patent hereinabove identified.Suffice it for the purpose of this application to state that in thephase comparator and up-down separator 81 output signal voltages ofgeneral pulse characteristics are provided and appear on the outputconductors 83 and 85 as pulses to be added or substracted in areversible counter mechanism. The counters are shown conventionally bythe counter components 86 and 87 which each comprise a plurality ofseparate indicator tubes and of which respectively and for illustrativepurposes may be considered as representing degrees of phase shift by thecounter 86 and tens, hundreds and thousands of degrees by the counter,schematically represented at 87 and thus forming a so-callcd verniercounter.

It is desirable to reset the vernier counter 87 to a zero state wheneverabsolute coincidence is achieved between the variable and referencephase signals at the assumed 1000:1 frequency dividers 65 and 67. Theresetting is achieved under the control of the zero crossing coincidencedetector conventionally shown at 39 in FIG. 3. In practice the signalsserving to control the zero crossing coincident detector 89 are fed byway of the input conductors 90 and 91, these representing,illustratively, an input determined by the reference and variable phaseoscillators 57 and 5-9 reduced in frequency by an order of 1000. Theinput signal supplied by way of the conductors is generally in pulseform and serves to provide a zero position indication once each cycle.The output signals from the zero crossing coincidence detectors 89'which control the vernier counter 87 then appear on the conductor 93.

Because of minor mechanical imperfections which are inherently presentin mechanically driven devices, despite their manufacture to the limitof mechanical perfection, disturbances which will herein be termedjitter frequently arise. While various causes may bring these effectsabout, one cause often is that due to the rotor shaft running veryslightly eccentric so that the amplitude of the generated output voltagewaves at the reference and variable frequencies, as appears on theconductors 53 and 55, may vary very slightly. The variation can be ineither or both amplitude and phase and even Where this is an extremelylow frequency change occurring over a long period of time it issufficient to cause certain imperfections in the operation unlesscompensation is provided. It is impossible in general to providemechanical compensation where the precision of manufacture of thetransducer component is the greatest obtainable. Accordingly, thisinvention makes provision for introducing with the signal generatingcircuitry an electrical compensator to avoid the detrimental effects ofmechanical power jitter.

One form of circuitry to provide for compensating for such pertubati-onshas been schematically represented and included in the block diagramillustration of FIG. 3 by the jitter corrector conventionally shown at94 and 95. Input signals to the jitter correctors 94 and 95 are providedby way of conductors 96 and 97. Produced correction signals from thejitter-compensators 94 and 95 are then su plied by way of conductors 98and 99, respectively, to the input of the oscillators 57 and 59.

The jitter corrector is shown and discussed in detail by makingreference to FIG. 2 of the drawings. In the jitter compensator signalvoltage is developed which is provided for the purpose of initiating avery minor phase modulation of the oscillator 57 or 59, as the case maybe, in order to compensate the phase error in the control of theoscillator.

In the jitter compensation circuit shown by FIG. 2, voltage of one ofthe two developed reference or adjustable frequencies is supplied at theinput terminal 101 and from there to the grid or control electrode 102of a tube 103. The tube 103 has its plate or anode voltage supplied froma source (not shown) connected to the terminal point 105, the sourcebeing poled positively relative to the tube plate. The tube cathode 106is connected to ground 137 through the cathode resistor 108.

An output voltage in phase with the input and fluctuating in amplitudein the same fashion as the input is obtained from the cathode followerand supplied through the coupling condenser 109 to the peak detectorcircuit which comprises the diodes 110 and 111 with resistor 112connected across diode 110. This provides peak detection of the inputamplitude fluctuations and the amplitude-varying signals are suppliedthrough the integrating circuit provided by the capacitors 113 and 114between which is the resistor 115 to be supplied for amplification tothe input grid or control electrode 117 of the tube 118. There is thusprovided, as the input voltage,

fluctuations occurring at the rate of change of the input signalamplitude.

The electronic system here described is generally of limited sensitivityto amplitude but extremely sensitive to phase changes. The jittercircuit above described makes use of the fact that amplitudefluctuations are present through the amplitude detection of the envelopeof the transducer output, which is put through a phase shifting and gaincontrol circuit.

In the diagrammed circuit tube 118 is supplied with plate voltage fromterminal through plate resistor 119. Cathode bias is derived throughcathode resistor 119'. Output voltage from the tube is available at boththe tube plate and cathode or across resistor 119 and 119'. The voltagesare then supplied to the conductors of a double-pole, double-throwswitch 120 to be made available in an output conductor 98 either throughcapacitor 121 or resistor 122 and through resistor 123 and 124 to thelatter of which the output coaxial conductor is adjustably connected.

As can be seen clearly from FIG. 2, the ouput available on the conductor98 (or 99) is then applied as a control voltage to control the amplitudeof the oscillator 57 (or 59 as the case may be). The control is appliedas a modulation of the oscillator in well-known fashion and need not beexplained in further detail.

Similar jitter compensation is provided for both the reference andvariable channels and therefore a single unit is diagrammed but separateunits are used in each channel as clearly shown by FIG. 2.

Many and varied modifications may be made to the circuitry here claimedand described. Therefore, it is believed that the claim hereinafterappended should be considered broadly and limited only insofar as priorart limitations necessitate it.

What is claimed is:

In an encoding system having a transducer means for generating recurringelectrical signals for indicating a relative angular position, saidsignals being subject to phase and amplitude modulation caused by minormechanical imperfections in said transducer, jitter circuit means forcompensating for said modulation, said jitter circuit means comprisingamplitude detecting means for detecting the amplitude variations of theenvelope of said modulated signals, cathode follower amplifier means forcoupling said modulated signals to said detecting means, means forintegrating said detected envelope, and control means responsive to saidintegrated signal reaching a predetermined level for differentiatingsaid integrated signal to provide a compensating signal whichcompensates for said phase and amplitude modulation; said control meanscomprising a cathode follower differentiating amplifier, double-pole,double-throw switching means for detecting the output from saiddifierentiating amplifier, either from said cathode or from the plate ofsaid amplifier, gain control means in series with the output of saiddifferentiating amplifier for controlling the amplitude of saidcompensating signal, and resistor means in said differentiating circuitfor controlling the phase of said compensating signal.

References Cited by the Examiner UNITED STATES PATENTS 2,222,172 11/1940Dimmick 328- X 2,227,906 1/1941 Kellog 328150 X 2,410,000 10/1946Anderson 325323 X 2,571,650 10/1951 Atwood.

2,647,238 7/1953 Bailey.

2,834,883 5/1958 Lubokk 328151 2,924,769 2/1960 Harriman et al. 324-103X ARTHUR GAUSS, Primary Examiner. S. D. MILLER, Assistant Examiner.

